Add codeclimate button to README

[alexanderdean]

[![Code Climate](https://codeclimate.com/github/snowplow/snowplow-javascript-tracker
.png)](https://codeclimate.com/github/github/snowplow/snowplow-javascript-tracker)

[alexanderdean]

https://codeclimate.com/github/snowplow/snowplow-javascript-tracker/badges

[alexanderdean]

Fred Blundun (@fblundun) - I am moving this back into 1.0.0, don't worry I will update the CHANGELOG...

[alexanderdean]

Done in 1.0.0, closing